The invention relates generally to the field of imaging and display devices.
Modern sensitive imaging systems are designed to detect objects emitting low levels of radiation. Medical diagnosis is a common application for such sensitive imaging systems. For example, a medical procedure may distribute low levels of radiation in an organ to allow imaging of the organ. Use of low radiation levels minimizes damage to organs and surrounding tissue.
One problem with sensitive imaging systems designed to detect low levels of radiation is that the typically weak signals used in such systems may be lost in even small amounts of noise. Thus, reduction of noise is an important consideration in these systems. One example of an imaging system often used in medical applications is an amorphous silicon, (a-Si) image sensor. Amorphous silicon sensor arrays are well-known devices for imaging incident energy (see R. A. Street et al. xe2x80x9cAmorphous Silicon Arrays Develop a Medical Imagexe2x80x9d, IEEE Circuits and Devices, July 1993, pp. 38-42 and hereby incorporated by reference).
Traditional sensor systems include a two dimensional array of pixels. Address lines running across the array of pixels are used to receive information from or transfer information to each pixel in the array of pixels. The address lines communicate the output of the array of pixels one line at a time. Typically, such output is accomplished by asserting a gate line resulting in the parallel output of data along a plurality of data outputs lines. Fluctuations in voltage and other noise artifacts that affect the entire pixel array at the time of assertion of a gate line can cause noise in information transmitted by the data lines during the voltage fluctuations. The fluctuation or noise artifact results in a line of noise across the constructed image.
Typically, an amorphous silicon sensor system is formed using a two dimensional arrangement of perpendicularly arranged address lines with individual sensors or pixels at the intersection. When used in X-ray detection, each pixel may include a scintillation layer such as a phosphor converter or an X-ray photosensitive photoconductor, that generates visible light from the non-visible radiation being detected. In some designs, a detector in the pixel captures some of the visible light and converts the light photons to free electrons. A capacitor in the sensor stores the electrons.
A control circuit selectively and independently discharges each of the capacitors. The amount of discharge from each capacitor represents the amount of incident light reaching a corresponding pixel. By monitoring the outputs of the capacitors, a representation or image of the organ or object being imaged may be created.
Address lines, typically metallization lines, communicate information between the pixels and the control circuit. Each addressing line runs straight across the array. For purposes of this invention, an addressing line is defined to include a gate line used to address or transmit information to a pixel, a bias line used to bias the transistors in a pixel, as well as a data output line which may be used to read out or transfer information from a pixel.
The above describes sensor systems that are susceptible to generating line correlated noise when a fluctuation occurs across the sensor system. Thus an improved sensor design is needed.
When time dependent changes occur to an addressing line or to an entire sensor array, for example a fluctuation in bias voltage across the entire sensor array, or capacitive coupling between different address lines, the time-dependent fluctuation may appear as line-correlated noise in the image sensor system output. For purposes of this invention, line-correlated noise is an undesirable signal artifact that causes an undesirable change across a line in an image constructed from the sensor output. The noise problem is particularly acute in imaging systems designed to detect low level signals. In such systems, even small amounts of noise can produce a substantial drop in the signal to noise ratio.
Human vision is very sensitive to line-correlated noise. Thus, at a given noise level, most observers find a level of noise distributed as a line in a visual field to be significantly more noticeable than the same level of noise randomly distributed. It is generally accepted that line-correlated noise must substantially lower in amplitude than random pixel noise in order to reach the same level of noticeability.
In order to reduce the amount of line-correlated noise, the present invention redistributes the noise by reconfiguring address lines to address or receive data from pixels in at least two different pixel lines. Typically, the two pixel lines are two parallel or horizontal lines of pixels in an array of pixels. In one embodiment of the invention, a first segment of an address line addresses pixels in a first pixel line parallel to the address line and positioned on a first side of the address line. A second segment of the address line addresses pixels in a second pixel line parallel to the address line and positioned on a second side of the address line. Using each address line to address pixels in different pixel lines redistributes noise to reduce the line correlation of the noise. In other embodiments of the invention, the address line is xe2x80x9csteppedxe2x80x9d such that a first segment of the address line runs between a first and a second pixel line (either row or column) while a second segment of the address line runs between a second and a third pixel line. Stepping the line allows a single address line to address different pixel lines to redistribute the noise and reduce the line correlation.